Field of the Invention
The invention relates to a heterojunction field-effect transistor or high-electron mobility transistor (HEMT) comprising a semiconductor structure made up of superposed layers deposited on a substrate, the superposed layers being composed of semiconductor materials having hexagonal crystal structures, of the Ga(1-p-q)Al(p)In(q)N type, where p and q may be comprised between 0 inclusive and 1 inclusive, the sum p+q being lower than or equal to 1, and the pair {p,q} being specific to each layer. The material of the substrate may for example be GaN, AlN, SiC, Si, diamond or sapphire.
This transistor will possibly be used in a monolithic microwave integrated circuit (MMIC) or in another circuit.
Description of the Related Art
A HEMT (also referred to as a HFET for heterostructure field-effect transistor or a MODFET for modulation-doped field-effect transistor) typically comprises:                a channel layer, made of a first semiconductor material, for example GaN;        a barrier layer on the channel layer, made of a semiconductor material with a wider bandgap than the first material, and a lower electron affinity than the first material, for example AlN or AlGaN;        a gate electrode forming, with the barrier layer, a Schottky junction; and        a source electrode and a drain electrode on either side of the gate electrode.        
It is known to deposit, in the locations corresponding to the source and drain electrodes, before these electrodes are deposited, a layer of GaN doped with Si atoms by epitaxial growth. This layer of doped GaN makes it possible to design transistors with smaller dimensions. A masking layer is used to define these locations.
The documents K. Shinohara et al, “Electron Velocity Enhancement in Laterally Scaled GaN DH-HEMTs with fT of 260 GHz”, IEEE Electron Device Letters, Vol. 32, No 8, August 2011 and T. Fujiwara, “Low Ohmic Contact Resistance m-Plane AlGaN/GaN Heterojunction Field-Effect Transistors with Enhancement-Mode Operations”, Applied Physics Express 3 (2010) describe two examples of processes for fabricating HEMTs on a GaN substrate, comprising steps of localized epitaxial growth of silicon-doped layers.
Document US 2005/0258451, cited in the preliminary search report established for the French patent application the priority of which is claimed here, describes a HEMT obtained by depositing the GaN layer, doped with silicon, germanium or oxygen, inter alia, at a low-enough temperature to prevent mass transport toward the locations corresponding to the drain and source electrodes. Even though the choice of germanium doping is mentioned among other possibilities, a low-temperature is chosen in order to prevent drawbacks related to mass transport in the case of epitaxial deposition of silicon-doped GaN, namely the risk of morphology problems, especially SiN precipitates, nonuniform GaN layer heights and a mediocre reproducibility. Because of the absence of mass transport, the GaN is deposited with a relatively small thickness in order prevent it extending past the dielectric layer.
There is a need for a relatively simple process allowing the definition of the edges of the localized epitaxial layers to be improved.